RCGP chairwoman Dr Clare Gerada wrote the letter ahead of the Liberal Democrats Spring Conference which opens on Saturday in Gateshead, Tyne and Wear.
The letter was co-signed by seven other medical organisations including the Royal College of Midwives and the Chartered Society of Physiotherapy.
Liberal Democrat members will on Saturday be given the opportunity to decide which of a possible number of amendments will be voted on the following day.
GP and chairman of Shrewsbury Liberal Democrats Dr Charles West helped to table one possible motion, calling for the Bill to be dropped. If the motion is passed at conference it will become Liberal Democrat policy.
In her letter Dr Gerada urged the Liberal Democrats to reconsider their stance on the Bill.
She said: ‘As organisations representing over 100,000 professionals working in the NHS and providing frontline care to patients, we urge you to rethink your position and consider the full impact of these reforms on our ability to provide care to patients.
‘We call on your party, and the coalition government, to work with doctors, nurses and other health professionals to find an acceptable way forward in the best interests of our patients,’ she said.